The long range objective of this project is to identify and characterize plasmids from oral streptococci, and to study their possible contribution to the physiology, ecology and pathogenicity of this group of organisms. Current studies have focused on a tetracycline (Tc) resistance determinant carried by an animal isolate of S. mutans. This determinant could transfer by conjugation to certain strains of S. faecalis and S. mutans. Whereas the transfer was accompanied by the appearance of a plasmid (pDJ2) in the S. faecalis transconjugants, no plasmid was evident in the original S. mutans host. Thus, the location of the Tc resistance determinant in this strain remained in question. pDJ2 was used as a probe to identify the Tc resistance locus in S. mutans 19S. Purified plasmid was labeled by nick-translation and hybridized to electrophoretically separated HindIII fragments of total cellular DNA. A unique 12 kilobasepair fragment was identified in all the Tc resistant strains. This result indicates that the Tc resistance determinant is encoded on a similarly sized plasmid in both the original S.mutans host and all the transconjugants. No evidence was found for a chromosomal Tc resistance determinant.